High BMI Cuts RA Risk in Men

Is there an advantage to being overweight?

Action Points

Overweight or obese men, but not women, have a significantly lower risk of developing rheumatoid arthritis (RA), according to two large, Swedish, population-based studies.

Although smoking was a significant predictor of RA in both men and women, it did not alter the impact of overweight or obesity on the reduced risk of RA in men.

Being overweight or obese may protect against rheumatoid arthritis (RA), but only in men, a new nested case-control study suggests.

It found that being overweight or obese at enrollment was associated with a reduced risk of later development of RA in men (odds ratio 0.33; 95% CI 0.14-0.76), but not in women (OR 1.01, 95% CI 0.65-1.54).

The sex difference for the impact of body mass index (BMI) on the risk of RA found in the study "is of interest and intriguing," according to Carl Turesson, of Malmö Lund University, Sweden, and colleagues.

The analysis, published in Rheumatology, included data from two large prospective population-based health surveys. These surveys were the Maimö Diet Cancer Study (MDCS), carried out from 1991 and 1996 and included 30,447 men and women; and the Malmö Preventive Medicine Program (MPMP), conducted from 1974 to 1992 and included 33,346 subjects born from 1921 to 1949 (males) or from 1925 to 1938 (females).

Researchers identified individuals who developed RA at least one year after inclusion in either survey, up to Dec. 31, 2004.

From the MDCS, 172 individuals (36 men and 136 women) were diagnosed with RA. The median time from inclusion to RA diagnosis was 5 years. In the MPMP, there were 290 cases of RA (151 men and 139 women) with a median time to diagnosis of 12 years.

For each validated case, researchers selected four controls, matched for sex, year of birth, and year of screening, who were living in Sweden and free of RA.

Overweight was defined as having a body mass index (BMI) of 25-30 and obese as a BMI of over 30. Normal weight was considered a BMI of 18.5-25. Subjects who were underweight (a BMI of less than 18.5) were excluded from the study.

There was as similar relationship for men in the MPMP (OR 0.60; 95% CI 0.39-0.91). The OR in overweight or obese women in this survey was 1.37 (95% CI 0.86-2.18).

Smoking was a significant predictor of RA in both men and women. In an analysis of the impact of overweight/obesity on the risk of RA that was adjusted for smoking again found a significant association in men (adjusted OR 0.37 in the MDCS and 0.60 in the MPMP).

The patterns were also unchanged in analyses that were adjusted for other potential confounders including level of education, alcohol use, and socioeconomic status.

The authors noted that BMI is "a rather crude measure" of metabolic status and may reflect underlying associations with lifestyle habits such as physical activity and dietary habits.

Although the disease process may start long before the clinical onset of RA, it's unlikely that inflammation-associated weight loss in men explains the results, said the authors. For one thing, although the effect of overweight or obesity seemed strongest among those included in the MDCS 5 or less years before RA diagnosis, a similar pattern was noted among those in the MPMP who were included 13 or more years before their RA diagnosis.

Geographical factors may explain the discrepancy between the current study and several studies from the U.S. that reported an increased risk of RA in obese women. Differences in the distribution of BMI between Scandinavian and U.S. populations may have contributed to this difference.

Also, as the present study included middle-aged and older women, it could not detect a particular effect of high BMI in young women.

A high BMI often reflects increased abdominal obesity or increased visceral fat in men compared with women, suggesting that the observations in this study may be due to a protective effect of abdominal or visceral fat against the development of RA. Components produced by adipose tissue may play a role in the pathophysiology of RA, said the authors.

The relationship between hormonal factors associated with RA and body fat distribution require further investigation, they said.

Limitations of the study were that it didn't include longitudinal data on BMI in most cases, or data on anti-citrullinated protein antibody (ACPA) status after diagnosis, as such analysis wasn't routine when the study subjects were diagnosed. The study also included mainly caucasians of Scandinavian heritage so the results may not apply to other groups, and it lacked information on genetic factors, the effects of which on BMI and RA susceptibility could confound the results.

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